Malignant tumors need an efficient metabolic system to meet demands for energy and substrates necessary for cancer cells to successfully grow and metastasize. Indeed, metabolism of cancer cells has been a subject of intense research for several years. It has been established that cancer cells switch their metabolism from oxidative phosphorylation to glycolysis (called the “Warburg effect”) and, unlike most normal cells, they produce large amounts of fatty acids for plasma membrane rebuilding and energy via β-oxidation. A critical question arising from this observation concerns the origin of citrate, which is a primary substrate for fatty acid synthesis.
Normally, citrate is thought to be produced via the Krebs cycle in mitochondria or through glutamine reductive carboxylation. The former possibility is considered less likely in cancer cells, since reduced mitochondrial activity is one of the hallmarks of malignancy. Moreover, cells would need to have a mechanism that would truncate the Krebs cycle to allow for citrate accumulation; evidence for this has not been found. Instead, up to now glutamine has been considered the major source of citrate in cancer cells. While glutamine consumption is increased in cancer, its role in cancer metabolism does not involve exclusively citrate synthesis, but rather the supply of necessary nitrogen for amino acid synthesis. Interestingly, glutamine reductive carboxylation that may lead to citrate synthesis requires increased reverse activity of the Krebs cycle; this possibility remains questionable in light of overall decreased mitochondrial activity in cancer cells.
Additionally, it should be taken into account that the use of this pathway affects cellular metabolic (red/ox) balance, which presents problems for these cells.
Dittrich et al., “Prostate cancer growth and citrate levels”, Prostate Cancer and Prostatic Diseases (2012) 15, 278-282, studied the potential utility of assessing prostate cancer progression by measuring citrate levels in prostate cancer tissue. They concluded that low levels of citrate in a unit volume correlate with rapidly increasing PSA values, and, therefore, may be used as an indicator of fast-growing prostate cancer. They noted that tissue samples obtained at the time of biopsy may be evaluated for their citrate concentrations for the prediction of prostate cancer growth rates, allowing for the implementation of alternative treatment options and reducing overtreatment.
Mycielska et al., 205: “Expression of Na+-dependent citrate transport in a strongly metastatic human prostate cancer PC-3M cell line: regulation by voltage-gated Na+ channel activity”; J Physiol. 2005 Mar. 1; 563: 393-408, describe that normal prostatic epithelial cells have a K+ dependent efflux mechanism for citrate, whereas malignant prostatic epithelial cells have a Na+ dependent transporter system primarily for uptake of citrate.
Mazurek et al., 2010: Molecular origin of plasma membrane citrate transporter in human prostate epithelial cells. EMBO Rep. 2010 June; 11(6):431-7. Epub 2010 May 7, present the results of molecular cloning of a citrate transporter from human normal prostate epithelial PNT2-C2 cells. By using rapid amplification of cDNA by PCR, Mazurek et al. determined that the prostatic carrier is an isoform of the mitochondrial transporter SLC25A1 with a different first exon. They confirmed that pmCiC is a major citrate transporter expressed in the plasma membrane of normal human prostate PNT2-C2 cells and non-malignant prostate tissues. However, there is no indication that pmCiC could be used as a tumor marker or as a target for anti-cancer therapy.
Vamsi K. Kolukula et al.: “SLC25A1, or CiC, is a novel transcriptional target of mutant p53 and a negative tumor prognostic marker”, Oncotarget, 15 Mar. 2014, p. 1212, describe SLC25A1, or CiC and its potential function to promote tumorigenesis, but do not take into account the potential presence of pmCiC in the cells.
pmCiC and mitochondrial citrate transporter (mCiC) belong to the same gene family. Both proteins are responsible for citrate transport. They have a similar structure, but different physiological functions.
Even though pmCiC and mCiC are coded on the same chromosome they have different start codons and different first exon and intron. Therefore, they cannot be even considered as splice variants because this is not due to alternative splicing. They are two separate genes occupying in-part the same loci.
mCiC is localized in the inner mitochondrial membrane and is responsible for citrate transport from and to mitochondria. pmCiC, on the other hand, is trafficked to the plasma membrane and in the case of cancer cells, imports extracellular citrate into the cytoplasm.
Despite significant amino acid similarities, the two transporters function in a totally different manner. mCiC transports citrate in both directions in an antiport system (citrate out against malate or citrate in, or citrate in against malate or citrate out). pmCiC transports citrate in one direction in a symport system (in cancer cells citrate is transported with Na+ in the same direction).
mCiC is responsible for a proper functioning of mitochondria by facilitating citrate/malate exchange between the mitochondria and cytoplasm. Blocking mCiC will of course result in decreased citrate/malate content in the cytoplasm, but primarily will affect mitochondrial activity by increasing the intra-mitochondrial level of citrate. Localized in the plasma membrane, pmCiC is responsible for citrate uptake from the extracellular space. Blocking or opening of the pmCiC will have no direct bearing on mitochondrial activity (quite the opposite to mCiC, long-term blocking of pmCiC will result in an increase in mitochondrial activity). pmCiC inhibition will only decrease or increase the cytosolic level of citrate.
To summarize, mCiC and pmCiC are two different proteins belonging to the same gene family. They transport citrate but their mode of action, function, localization and physiological meaning for the cell are completely different. Most importantly, mCiC is ubiquitously expressed in the mitochondrial membrane of all cells, whilst pmCiC expression is mainly restricted to cancer cells. Moreover, any specifically designed modulators of pmCiC will not affect mCiC as they will not penetrate the cell membrane.